The adrenal gland is a common site of cancer metastasis. Surgery remains a mainstay of treatment for solitary adrenal metastasis. For patients who cannot undergo surgery, radiation is an alternative option. Stereotactic body radiotherapy (SBRT) is an ablative treatment option allowing larger doses to be delivered over a shorter period of time. In this study, we report on our experience with the use of SBRT to treat adrenal metastases using CyberKnife technology. We retrospectively reviewed the Winthrop University radiation oncology data base to identify 14 patients for whom SBRT was administered to treat malignant adrenal disease. Of the factors examined, the biological equivalent dose (BED) of radiation delivered was found to be the most important predictor of local adrenal tumor control. We conclude that CyberKnife-based SBRT is a safe, non-invasive modality that has broadened the therapeutic options for the treatment of isolated adrenal metastases.

e21114 Background: Adrenalectomy achieves long term survival in ~ 25% of patients with solitary adrenal metastasis (SAM). Stereotactic radiosurgery (SRS) represents a potential non-surgical alternative to this same endpoint, but reported local control rates vary from 45% to 96%. SRS delivers a range of biologic effective doses (BED) dependent upon fraction size, number and schedule. We herein review our experience with SRS for adrenal metastasis to evaluate the interaction between BED and local adrenal tumor control. Methods: The Winthrop SRS data base was queried to identify 14 patients undergoing SRS for malignant adrenal tumors from 2006 to 2011. Patients were characterized by their primary tumor site, stage at diagnosis and the time to adrenal involvement. BED was calculated for each patient. Local adrenal tumor control and durability of response were also recorded. Results: Calculated BEDs ranged from 4,667 cGy to13,000 cGy. Of the factors examined, BED was the single most important predictor of local adrenal tumor control. According to best adrenal tumor response, mean BEDs were: 10,053 cGy for radiographic regression of disease; 8,115 cGy for stable disease, and 6,667 cGy for progression of disease (p = 0.0465, by one-tail test). No solid tumor patient responded to SRS with BED < 8,800 cGy and no patient immediately progressed through SRS with BED > 6,667 cGy. Duration of adrenal tumor control also correlated with calculated mean BED which was: 9,676 cGy for never locally failing (n=6) and 7,600 cGy for ever locally failing tumors (n=5). However, eventual local treatment failure was seen in one of three patients receiving even the highest calculated BED (13,000 cGy). Conclusions: Because eventual local failures are observed with SRS even at the highest examined BED, adrenalectomy rather than SRS should remain the standard of care for patients with potentially curable SAM. SRS remains an acceptable alternative to adrenalectomy for palliation of symptomatic adrenal tumors or for the definitive treatment of SAM in non-surgical candidates. When given for these purposes, our data support the use of SRS regimens delivering a minimum BED of 8,800 cGy.